The design life of a component or product is the period of time during which the item is expected by its designers to work within its specified parameters; in other words, the life expectancy of the item. It is the length of time between placement into service of a single item and that item's onset of wearout.
The design life of components and products differs from the items' "mean time between failure (MTBF), in that MTBF is a measure of the rate of occurrence of random failures in time where these failures are not due to a wear-out mechanism. For example, the MTBF of a device may be 100,000 hours and the design-life is 20,000 hours. In this example, across the population of products, one failure will occur, on average, every 100,000 population operating hours. (100,000 units operating for 1 hour each = 100,000 population operating hours.) None of these units will ever approach reaching 100,000 operating hours as each one will fail due to wear-out and be replaced by a new unit. "Aluminum electrolytic capacitors, "fans, and "batteries are classic examples of components that will fail due to wear-out well before they could individually achieve the operating time indicated by their MTBF.
Another use of the term design life deals with consumer products. Many products employ design life as one factor of their differentiation from competing products and components. A "disposable camera is designed to withstand a short life, whilst an expensive "single-lens reflex camera can be expected to have a design life measured in years or decades. (Clearly in this example there are other differentiators.)
Some products designed for heavy or demanding use are so well-made that they are retained and used well beyond their design life. Some "public transport "vehicles come into this category, as do a number of "artificial satellites and "spacecraft.
In general, entry-level products—those at the lowest end of the price range fulfilling a certain specification—will tend to have shorter design lives than more expensive products fulfilling the same function, since there are savings to be made in using designs that are cheaper to implement, or, conversely, costs to be passed onto the customer in engineering to provide a safe margin leading to an increased working life. This "economic truism leads to the phenomenon of products designed (or appearing to be designed) to last only so long as their warranty period.
Design life is related to but distinct from the concept of built-in obsolescence. The latter is the somewhat more nebulous notion that products are designed so as to become "obsolete—at least in the eyes of the user—before the end of their design life. Two classic examples here are "digital cameras, which become genuinely obsolete as a result of the very rapid rate of technological advances, although still in perfect working order; and non-digital "cameras, which are perceived as obsolete after a year or so as they are no longer "the latest design" although actually capable of years of useful service.